Boost pressure increases as it CAN flow more air.
Same thing happens with my supercharged 4Runner - it ends up sucking through a straw with my snorkel so I opened up the intake with a marine application Deck Plate (6"). When closed I max out at 7 psi or so, with it off, 8.5psi.
It would seem that the stock box is fine for a 'stage 3' car (I hate stages...) but if you go with a bigger turbo then it can become a bottleneck of sorts. Albeit, not a huge restriction.
I think a well made and well engineered airbox is the only upgrade - an Injen style intake (and I only pick on them as it was the first to come to mind) doesn't offer any performance gains at all.